Transcript No Slide Title

Cenozoic Life
www.geo.ucalgary.ca/~macrae/timescale/time_scale.gif
Cenozoic Life History
• The Cenozoic was the time during which
– Earth’s present-day fauna and flora evolved
– trends established millions of years earlier
continued
• Fewer skull and jaw bones during the
transition
– from fish to amphibians
– and then to reptiles
– and finally to mammals
Good Fossil Records
• Cenozoic rocks are especially common
– in western North America
– also found along the Gulf and Atlantic coasts
– horses, rhinoceroses, rodents, rabbits, and
camels have very good fossil records
Changing Climatic Patterns
• Changing climatic patterns
– accompanied by shifting plant distributions
characterize the Tertiary
• During the Paleocene and Eocene
– mean annual temperatures were high
– abundant precipitation fell
– tropical to semitropical forests covered much
of North America
Leaf Structure
• Leaf structure can give information
about:
– Paleoprecipitation
– Paleotemperature
• Precipitation: drip tips
– Reduces fungal infections
– Reduces parasitic plant infections
• Temperature: degree of leaf
serration
– Correlation between big smooth
leaves (entire margin) and warm
climates
• Not clear why
Plant leaves as Climatic Indicators
• Climatic trends for four areas in North
America
– based on the
percentages
of plant
species with
entire margin
leaves
Major Climatic Change
• A major climatic change took place at the end of
the Eocene
– when mean
annual
temperatures
dropped 7
degrees C in
3 million
years
Climatic Change
• Since the Oligocene
– mean annual temperatures have varied
somewhat worldwide
– overall have not changed much in the middle
latitudes except during the Pleistocene
Decrease in Precipitation
• A general decrease in precipitation
– over the last 25 million years
– in the midcontinent region of North America
• As the climate became drier vast forests of
the Oligocene
– gave way first to savanna conditions
• grasslands with scattered trees
– and finally to steppe environments
• short-grass prairie of the desert margin
Mammal Diversification
• With the demise of dinosaurs and their relatives
– mammals adaptively radiated
– remarkable diversification continued throughout the
Cenozoic Era
• The Age of Mammals had begun
Marsupial Mammals
• Marsupial mammals give birth to live young
– born in a very immature, almost embryonic condition
– undergo further development in the mother's pouch
• Marsupials probably migrated to Australia,
– the only area in which they are common today,
– via Antarctica before Pangaea fragmented completely
South American Marsupials
• Quite widespread in South America until a
few millions of years ago
• Most South American marsupials died out
– when a land connection was established
between the Americas
– and placental mammals migrated south
• Now the only marsupials
– outside Australia and some nearby islands are
species of opossums
Placenta
• Like marsupials, placental mammals give
birth to live young,
– but their reproductive method
– differs in important details
• In placentals, the amnion of
the amniote
– has fused with the walls of
the uterus
– forming a placenta
Marsupial Placenta Less Efficient
• Nutrients and oxygen flow
– from mother to embryo through the placenta
– permitting the young to develop much more
fully before birth
• marsupials also have a placenta
– but it is less efficient
– explaining why their newborn are not as fully
developed
Success of Placental
Mammals
• A measure of the success of placental
mammals
– is partially related to their method of
reproduction
• More than 90% of all mammals
– fossil and extinct, are placentals
Fossil Record of Horses
• With the possible exception of camels,
– no group of mammals has a better fossil
record
• horse fossils are so common,
– especially in North America
– where most of their evolution took place
– that their overall history and evolutionary
trends are quite well known
Horse Evolution
• Some evolutionary
trends in horses
– an increase in size
– lengthening of the
limbs
– reduction in the
number of toes
– development of highcrowned teeth with
complex chewing
surfaces
Trends in Horses
• Size increase
• Legs and feet become longer: for running
• Lateral toes reduced to vestiges
• Straightening and stiffening of the back
• Adaptations for grinding abrasive grasses
• Larger, more complex brain
Horse Evolution Branched
• Horse evolution proceeded along two
distinct branches
• One led to three-toed browsing horses
– all now extinct
• The other led to three-toed grazing horses
– and finally to one-toed grazers
• The appearance of grazing horses
– with high-crowned chewing teeth
– coincided with the evolution and spread of
grasses during the Miocene
Low- and High-Crowned Teeth
• Once grasses had evolved
– many hoofed mammals became grazers
– developed high-crowned, abrasion-resistant teeth
• Low-crowned teeth
– typical of many
mammals with varied
diets
• High-crowned,
cement-covered
chewing teeth
– are adapted for
grazing
Mammals of the Ice Age
• The most remarkable aspect
– of the Pleistocene mammalian fauna
– is that so many very large species existed
• Mastodons, mammoths, giant bison,
– huge ground sloths, immense camels, beavers
2 m tall
– present in North America
Cooler Conditions—Larger Sizes
• Many smaller mammal species also
existed
– but obvious trend among Pleistocene
mammals was large body size
• Perhaps this was an adaptation
– to the cooler conditions
• Large animals have less surface area
– compared to their volume
– thus retain heat more effectively than do
smaller animals
– (but what about big dinosaurs?)
Frozen Mammals
• Some of the world's best-known fossils
– come from Pleistocene deposits
– frozen mammals found in Siberia and Alaska,
– such as mammoths, bison, and a few others
• These extraordinary fossils,
– although very rare,
– provide much more information than most
fossils do
Frozen Baby Mammoth
• Frozen baby mammoth found
– in Siberia in 1971
– 1.15 m long
and 1.0 m tall
– had a hairy
coat
• Recovered from
permafrost
Pleistocene Extinctions
• Extinctions have occurred continually
– at times of mass extinctions, Earth's biotic
diversity sharply declined
– as at the ends of the Paleozoic and Mesozoic
eras
• In marked contrast,
– the Pleistocene extinctions were rather
modest
– did have a profound effect on genera of large
terrestrial mammals
Extinctions
(1) What caused these extinctions?
(2) Why did these extinctions eliminate mostly
large mammals?
(3) Why were extinctions more severe in
Australia and the Americas?
• No completely satisfactory explanation
exists
– but two competing hypotheses are currently
being debated
Extinction Hypotheses
• Rapid climatic changes at the end of the
Pleistocene
• Prehistoric overkill
– holds that human hunters were responsible
Climate and Vegetation Changes
• Rapid changes in climate and vegetation
– occurred over much of Earth's surface during
the Late Pleistocene
– as glaciers began retreating
• In North America and northern Eurasia
– conifer and broadleaf forests replaced opensteppe tundras
– warmer and wetter conditions prevailed
Climate and Vegetation Changes
• The southwestern U.S. region changed
– from a moist area with numerous lakes
• where saber-toothed cats, giant ground sloths, and
mammoths roamed
– to a semiarid environment unable to support a
diverse large mammalian fauna
Why Didn't
Large Mammals Migrate?
• Rapid changes in climate and vegetation
– can certainly affect animal populations
– but the climate hypothesis presents several
problems
• First, why didn't the large mammals
migrate to more suitable habitats as the
climate and vegetation changed?
– many other animal species did
Mammal Migration in Europe
• For example, reindeer and the Arctic fox
– lived in southern France during the last
glaciation
– migrated to the Arctic when the climate
became warmer
Argument Against
the Climatic Hypothesis
• The second argument against the climatic
hypothesis
– is the lack of correlation between extinctions and the
earlier glacial advances and retreats throughout the
Pleistocene Epoch
• Previous changes in climate
– were not marked by episodes of mass extinctions
Arrival of Humans
• Proponents of the prehistoric overkill
hypothesis
– argue that the mass extinctions in North and
South America and Australia coincided closely
with the arrival of humans
• Perhaps hunters had a tremendous impact
– on the faunas of North and South America
– about 11,000 years ago because the animals
had no previous experience with humans
Arrival of Humans
• The same thing happened much earlier in
Australia soon after people arrived about
40,000 years ago
• No large-scale extinctions in Africa and
most of Europe
– because animals in those regions had long
been familiar with humans
Extinctions on Oceanic
Islands
How could a few hunters decimate so
many species of large mammals?
• Humans have caused major extinctions on
oceanic islands
– in a period of about 600 years after arriving in
New Zealand, humans exterminated several
species of the large, flightless birds called
moas
Hunters Concentrate
on Small Animals
• A problem is that present-day hunters
concentrate on smaller, abundant, and less
dangerous animals
– remains of horses, reindeer, and other small
animals are found in many prehistoric sites in
Europe
– whereas mammoth and woolly rhinoceros
remains are scarce
Other Arguments
• Few human artifacts are found among the
remains of extinct animals in North and
South America
– and there is usually little evidence that the
animals were hunted
• Countering this argument
– is the assertion that the impact on the
previously unhunted fauna
– was so swift as to leave little evidence
Multiple Reasons
• The reason for the extinctions
– of large Pleistocene mammals is still unresolved
• It may turn out that the extinctions
– resulted from a combination of different circumstances
• Populations that were already under stress from
climatic changes
– were perhaps more vulnerable to hunting
– especially if smaller females and young animals were
the preferred targets
How do we know we had ice
ages?
• Geologic evidence:
– Moraines
– Poorly sorted sediments
– Scratched rocks
Moraines
• Most important glacial deposits
– chaotic mixtures of poorly sorted sediment
deposited directly by glacial ice
– An end moraine is deposited
– when a glacier’s terminus remains stationary for
some time
Mt. Cook, 1999
Recessional Moraine
• If the glacier’s terminus
– should recede and then stabilize once again
– another end
moraine forms
– known as a
recessional
moraine
Glacial Features
• Features seen in areas once covered by glaciers
• glacial polish
– the sheen
• striations
– scratches?
Devil’s Postpile
National
Monument,
California
Glacial Sediment
• Glaciers typically deposit poorly sorted
nonstratified sediment
How do we know how cold it got?
• Isotopes of oxygen!
• Oxygen:
– All isotopes have 8 protons
– Most common isotope has 8 neutrons
– Extremely rare: 9 neutrons
– Rare but detectable: 10 neutrons
– (why are 10 and 8 more common than 9?)
Oxygen Isotope Ratio
18O
http://www.ngdc.
noaa.gov/paleo/
ctl/about.html
16O
16O
18O
Oxygen Isotope Ratio
16O
18O
http://www.ngdc.
noaa.gov/paleo/
ctl/about.html
16O
18O
Oxygen Isotope Ratio
16O
18O
http://www.ngdc.
noaa.gov/paleo/
ctl/about.html
16O
18O
Onset of the Ice Age
QUATERNARY
CENOZOIC ERA
Today
10,000
~2 Ma - Northern Hemisphere
~30 Ma - West Antarctic
~45 Ma - East Antarctic
60 Ma
Cenozoic
Glaciations
Why the Icehouse?
• Long-term climate drivers:
– Plate tectonics
• Opening/closing of seaways
– Ocean currents are our heat and AC
• Uplift and erosion of mountains
– Weathering reduces atmospheric
CO2
– Life: catastrophic evolution of new
capabilities
– O2
– Astronomical drivers
• Other bodies (moon, sun) pull on the
Earth, changing its distance to the sun
Milutin Milankovic